1. Field of the Invention
This invention relates broadly to modular filter elements, filter apparatus comprising such filter elements and to methods of making and using such elements and apparatus. More particularly, it concerns modules that may be used to form rigid filter beds by positioning a multiplicity of the modules on a support base, methods of forming the new modules and methods of forming filter beds from the modules.
2. Description of the Prior Art
There are a number of filtering operations in which a filter cake containing much fluid is formed and which presents special handling problems. For example, in the operation of wastewater treatment plants this occurs in the process of dewatering sludge as a step in disposing of it. In conventional sludge dewatering operations, the sludge influent to the dewatering step will contain typically less than 2% solids. In order to handle the sludge as a "dry product" for disposal, the sludge should contain at least about 8% solids.
Sludges generated in water treatment plants and wastewater plants can be divided into a few basic types, e.g., metal hydroxide, metal carbonate or alum sludge (chemical), waste and wastewater sludge, aerobic domestic sewage sludge (bio-mass) and anaerobic digested domestic sewage sludge (bio-mass). The present invention relates to new devices and methods capable of handling all of such classes of sludge.
Conventional filter apparatus for sludge dewatering is an open tank containing a bed of loose sand supported on a lower bed of gravel which, in turn, is supported upon the floor of the tank, usually made of concrete. This type equipment is referred to in the trade as a sand bed.
As water drains from the influent in such equipment, a mushy filter cake builds up on the top of the sand layer until the cake contains about 6% solids. At such point, cracks start to occur in the cake and these grow as more water is removed. Depending upon the amount of time the sludge charge is allowed to remain in the equipment, it can be dewatered up to about 12-15% solids. At about 6-8% solids content, the sludge is removed from the sand bed by shovels, hoes or other scraping or scooping devices. Usually a thin layer of sludge will remain on the top of the sand bed and will hinder the dewatering of the next batch of influent. Rakes or scrapers are often used to remove this sludge residue, but this can result in loss of sand eventually requiring replacement of the sand bed. Furthermore, this type sludge dewatering is undesirably slow and requires large area filters to attain needed throughput. Land acquisition costs for such units, therefore, can be high.
Another type of sludge dewatering apparatus uses a rigid bed or plate of solid particles bound in a matrix of cured resin in place of the conventional sand bed (see U.S. Pat. No. 4,208,288). The broad idea of forming filter elements by combining aggregate particles with a resin matrix to form a rigid filter media (RFM) was known in the filter art well prior to that patent (see U.S. Pat. No. 3,538,020).
Outside the field of sludge dewatering, it has also been known to form filter apparatus by creating a filter surface by placing rigid porous plates side-by-side in a tank and supported above the bottom of the tank to provide fluid removal space beneath the resulting filter bed. This type of equipment was developed as early as 1918 for use in filtering corrosive chemicals (see U.S. Pat. No. 1,277,832). Such prior knowledge was advanced by new forms of filter elements and devices disclosed in the related U.S. Pat. No. 4,431,549. This present invention provides yet further substantial improvements over the filter elements and devices disclosed in that recent patent.
While the invention is described with particular reference to water and wastewater sludge dewatering, the new elements, apparatus and methods may be effectively used for the filtration of a wide spectrum of other fluid suspensions, especially those that form filter cakes that are difficult to handle during and after their formation.